| Receptor-independent signaling mechanisms
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| Some low-molecular-weight signaling molecules traverse the cell membrane
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Although most extracellular signals mediate their effects via receptor ligation of either cell surface or cytoplasmic receptors, some low-molecular-weight signaling molecules are able to traverse the plasma membrane and directly modulate the activity of the catalytic domains of transmembrane receptors or cytoplasmic signal transducing enzymes (see Fig. 38.1). For example, nitric oxide (NO), which has been postulated to have a variety of functions including signaling the relaxation of smooth muscle cells in blood vessels, can stimulate guanylate cyclase, leading to the generation of the second messenger, cGMP. Patients with angina are treated with glyceryl trinitrate, which is converted to NO, resulting in relaxation of the blood vessels. The consequent improvement in oxygen delivery to the heart muscle eases the pain that was caused by inadequate blood flow to the heart.
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